Step-by-step explanation:
If a cell needs to spend energy to accomplish a task, the ATP molecule splits off one of its three phosphates, becoming ADP (Adenosine di-phosphate) + phosphate. The energy holding that phosphate molecule is now released and available to do work for the cell. ... When it's run down, it's ADP.
ATP is converted into ADP and ADP can be changed back to ATP by the addition of a phosphate group.